Standardize shell execution on unified exec (#39757)

## What changed

- Remove the legacy `shell_command` handler and runtime, leaving `exec_command`
  and `write_stdin` as the shell execution tools.
- Treat legacy `shell_command` model metadata as `unified_exec`, and normalize
  legacy user opt-outs so they do not disable command execution. Managed feature
  requirements and `shell_tool` can still disable it.
- Preserve shell approvals, sandboxing, zsh-fork support, and output truncation
  through the unified execution path.

## Testing

- Cover legacy configuration and model-metadata compatibility.
- Exercise unified shell execution, approvals, truncation, and `apply_patch`
  serialization across the app-server and core test suites.

GitOrigin-RevId: 5c2fd6164fc3519cdae4944cb9db276b8467311c

scripts/mock_responses_websocket_server.py

@@ -14,9 +14,9 @@ HOST = "127.0.0.1"
 DEFAULT_PORT = 8765
 PATH = "/v1/responses"
 
-CALL_ID = "shell-command-call"
-FUNCTION_NAME = "shell_command"
-FUNCTION_ARGS_JSON = json.dumps({"command": "echo websocket"}, separators=(",", ":"))
+CALL_ID = "exec-command-call"
+FUNCTION_NAME = "exec_command"
+FUNCTION_ARGS_JSON = json.dumps({"cmd": "echo websocket"}, separators=(",", ":"))
 
 ASSISTANT_TEXT = "done"